Hello Dan,
Your commission looks amazing and dynamic!
I've been going through anatomy myself for a while now. I also felt I was progressing very slow. I appreciate that you kept it positive by saying your practice is still continuous, and I think having that mindset will take you far : )
my tip is to try to simplify and group muscles together into simpler shapes, after you have watched and practiced with Marc's anatomy lectures. Perhaps watching other professional artists explain their take on anatomy will help with that. That way you could rely less on reference and draw realistic poses out of your imagination more.
Hope to meet you at the end of the anatomy rabbit hole!